Adopt eslint-plugin-import-x to fix resolver loading (/typescript + /legacy)

[kpal81xd]

Wave 1 of the v3 rollout (#48) surfaced two resolver bugs that hit consumers on a normal install.

Bug A — /typescript: "invalid interface loaded as resolver"

eslint-plugin-import loads resolvers by name, relative to each linted file. The bundled eslint-import-resolver-typescript isn't always hoisted to the consumer's top level, so the lookup fails and falls back to require('typescript') (the compiler) → "invalid interface". A pilot (observer) saw 55 false import/* errors.

Bug B — /legacy: valid package subpaths flagged

/legacy enabled import/no-unresolved with no resolver, so the default node resolver (no exports-map support) flagged valid subpath imports like @playcanvas/eslint-config/legacy — the migration's own import line.

Fix — switch to eslint-plugin-import-x

Vanilla eslint-plugin-import (stale since mid-2025) only resolves resolvers by name from the linted file, which is the root of both bugs. eslint-plugin-import-x (the maintained fork) supports import-x/resolver-next, which lets the config pass the resolver object directly (import { createTypeScriptImportResolver }) — so resolution never depends on install hoisting. This is the setup eslint-import-resolver-typescript's own README blesses for flat config.

Per-tier namespace, by design:

• /typescript — native import-x: rules are import-x/*, import-x/resolver-next: [createTypeScriptImportResolver({ alwaysTryTypes: true })]. A fresh strict tier (not a drop-in), so the new namespace is the clean choice; consumers that override import rules use import-x/*.
• /legacy — import-x under the import namespace so rule names stay import/* and existing v2 consumer overrides keep working (preserves the drop-in promise for JS consumers like the engine). import-x/resolver-next: [createNodeResolver(), createTypeScriptImportResolver()] — node stays primary, exports-aware resolver only catches subpaths node misses.
• Bumps typescript-eslint → ^8.60.1 (import-x's @typescript-eslint/utils ^8.56 peer); drops eslint-plugin-import.

TypeScript import-rule tuning (/typescript only)

• Disable the TS-redundant, resolver-heavy rules: import-x/named, import-x/namespace, import-x/default, import-x/no-named-as-default-member → off. The type checker enforces these more accurately, and they're the slowest import rules — a real win on the large TS repos in later waves. import-x/no-unresolved stays on (catches path typos / asset imports).
• Type-import hygiene (pairs with @typescript-eslint/consistent-type-imports): import-x/consistent-type-specifier-style: ['error', 'prefer-top-level'], import-x/no-duplicates: ['warn', { prefer-inline: false }] — keep type-only imports as separate top-level import type statements.
• alwaysTryTypes: true on the resolver so @types-only packages resolve (matches /legacy).

(/legacy keeps named/namespace/default on — it lints JS+JSDoc, where the type checker isn't doing that work.)

Validation

• Build (tsc), prettier --check, self-lint (dogfoods /typescript): green — resolves an exports subpath (eslint-config-prettier/flat) + the typescript-eslint exports package, zero import-x/no-unresolved, zero "invalid interface". print-config confirms the per-tier namespaces and rule states.
• All entry points load; publint: all good.
• Pilot re-lint (observer/pcui-graph/canvas-mock) lands when they bump to beta.4. observer's import/no-unresolved: off override becomes import-x/no-unresolved: off (I'll handle it).

Target a 3.0.0-beta.4 cut from this so the held Wave 1 pilots can move off the buggy beta.3.